Airfoil flow receptivity to simulated tonal noise emissions

Abstract

On airfoils at low to moderate Reynolds numbers, trailing edge sound can have a feedback effect on the development of instability waves resulting in strongly periodic vortex shedding and tonal noise. This complex problem is decoupled in this study to investigate the flow response to deterministic noise emissions. The flow receptivity and response to tonal noise are examined using controlled steady and transient acoustic excitation. Experiments are performed on a NACA 0018 airfoil at a Reynolds number of 1.15×105 and an angle of attack of 5°, with a separation bubble on the suction side. Controlled acoustic perturbations are introduced via a loudspeaker. A combination of mean and time-resolved surface pressure and simultaneous high-speed particle image velocimetry measurements provides insight into the response of the bubble to simulated acoustic forcing. Amplification of shear layer perturbations in the fore portion of the separation bubble is observed only after some time delay from the onset of acoustic forcing. This places the receptivity region within the boundary layer upstream of the separation bubble, extending from the leading edge to just downstream of the suction peak. The acoustically excited perturbations in the receptivity region attain higher amplitudes than natural disturbances and are amplified significantly in the separation bubble, leading to a transient reduction in the separation bubble size. Results elucidate the ensuing transient response of the separation bubble, illustrating the flow dynamics expected during the initiation of the acoustic feedback loop during natural tonal noise emissions on an airfoil.